1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high power pulsed lasers, in the shock processing of solid materials, and, more particularly, to methods and apparatus for improving properties of solid materials by providing shockwaves therein. The invention is especially useful for enhancing or creating desired physical properties such as high cycle fatigue life, fretting fatigue life, hardness, strength, and fatigue strength.
2. Description of the Related Art
Old methods for the shock processing of solid materials typically involve the use of high explosive materials in contact with the solid, high explosive materials or high pressure gases used to accelerate a plate that strikes the solid to produce shockwaves therein. Such methods have several disadvantages. For example: (a) it is difficult and costly to shock process non-planar surfaces and complicated geometries, (b) storage and handling of the high explosive materials and high pressure gases pose a hazard, (c) the processes are difficult to automate and thus fail to meet some industrial needs and (d) high explosive materials and high pressure gases cannot be used in extreme environments such as high temperatures and high vacuum.
Shot peening is another widely known and accepted process for improving the fatigue, hardness, and corrosion resistance properties of materials by impact treatment of their surfaces. In shot peening, many small shot or beads are thrown at high speed against the surface of a material. However, the depth of treatment using conventional shot peening is typically only 0.004 to 0.006 inches deep. This depth is only 10–20 percent as deep as achieved with laser peening, and the surface enhancement of material properties with shot peening is much less effective.
Laser shock processing with coherent radiation has several advantages over what has been done before. For example, the source of the radiation is highly controllable and reproducible. The radiation is easily focused on preselected surface areas and the operating conditions are easily changed. This allows flexibility in the desired shocking pressure and careful control over the workpiece area to be shocked. Workpieces immersed in hostile environments, such as high temperature and high vacuum can be shock processed. Additionally, it is easy to shock the workpiece repetitively. This is desirable where it is possible to enhance material properties in a stepwise fashion. Shocking the workpiece several times at low pressures can avoid gross deformation, cracking, and spallation of the workpiece while non-planar workpieces can be shock processed without the need of elaborate and costly shock focusing schemes.
Laser peening (hereinafter referred to as laser shock processing) utilizes two overlays: a transparent overlay (usually water) and an opaque overlay, previously an oil based or acrylic based black paint. Tapes, such as black polyvinyl chloride or polyethylene tapes, have also been used successfully as the opaque overlay. During processing, a laser beam is directed to pass through the water overlay and is absorbed by the opaque overlay (black paint or tape), causing a rapid vaporization of the opaque overlay surface and the generation of a high-amplitude shockwave. The shockwave cold works the surface of the part as it propagates into the material and creates compressive residual stresses, which provide an increase in fatigue properties of the part. A workpiece is typically processed by processing a matrix of overlapping spots that cover the fatigue critical zone of the part.
The current laser processing of workpieces requires multiple re-applications of the opaque overlay, which require that the workpiece be manually removed from the laser processing station and recoated after several non-adjacent spots have been processed. When using paint, the old paint is sometimes removed before repainting the part or sometimes additional paint is simply added over the old paint. The repainting can require upwards of 12 to 15 paint cycles. Each cycle usually requires 15–20 minutes before the part can be returned to the processing station. This additional handling of the part for repainting will add as much as 50% to the cost of the processing in a production environment. In a similar manner, re-application of tape to overcome the problem of tape damage from the laser shot during processing adds significantly to the processing time and cost.
Other drawbacks have been found to be associated with using paints, both oil-based and water-based ones. These drawbacks stem from the need to dry paint prior to use thereof. Such drying adds to the process cycle time and/or increases system requirements in order to promote faster drying times. Furthermore, in the case of water-based paints, the paint can be rewet by the transparent overlay, such as flowing water, and may be eroded from the surface of the part before the laser beam is applied to the part. The removal of oil-based paints, not dissolved by a water overlay, must be achieved chemically and/or physically in a manner that essentially dissolves the paint or results in the flaking thereof. Additionally, there is no potential of relocating unused dried paint, in situ, to a new location where it may be used so as to reduce the total amount of paint to be applied. As such, recycling and/or reuse of paints is not practical, given typical removal methods.
A method of automatically applying the overlays in sequence with the laser system has been developed into an applicator system. This applicator system has, when used with the laser peening system, reduced the time of applying the overlay coatings and increased the throughput of the laser peen process. The opaque overlays used tend to be eroded by the transparent overlay during processing and have resulted in the generation of a weaker shockwave than achieved with tape or dry paint.
What is needed in the art is a laser shock process that utilizes an opaque overlay that does not have to dry, will not be eroded by the transparent overlay and yet allows for the generation of a strong shockwave that can be applied with the applicator system.